Media Gateway Control and the Softswitch Architecture

1
Media Gateway Control and the Softswitch
Architecture

• Chapter 6

2
Introduction

• Voice over IP
• Lower cost of network implementation
• Integration of voice and data applications
• New service features
• Reduced bandwidth
• Replacing all traditional circuit-switched
  networks is not feasible.
• VoIP and circuit-switching networks coexist
• Interoperation
• Seamless interworking

3
Separation of Media and Call Control

• Gateways
• Interworking
• To make the VoIP network appear to the circuit
  switched network as a native circuit-switched
  system and vice versa
• Signaling path and media path are different in
  VoIP systems.
• Media directly (end-to-end)
• Signaling through H.323 gatekeepers (or SIP
  proxies)
• SS7, Signaling System 7
• The logical separation of signaling and media

4
Separation of Media and Call Control

• A network gateway has two related but separate
  functions.
• Signaling conversion
• The call-control entities use signaling to
  communicate.
• Media conversion
• A slave function (mastered by call-control
  entities)
• Figure 6-1 illustrates the separation of call
  control and signaling from the media path.

5
Separation of Media and Call Control

• Advantages of Separation
• Media conversion close to the traffic source and
  sink
• The call-handling functions is centralized.
• A call agent (media gateway controller - MGC) can
  control multiple gateways.
• New features can be added more quickly.
• MGCP, Media Gateway Control Protocol
• IETF
• MEGACO/H.248
• IETF and ITU-T Study Group 16

6
Softswitch Architecture 1/2
7
Softswitch Architecture 2/2
SS7 Network
Internet
Signaling (SS7) Gateway
SCP
MGCP/MEGACO
Call Agent
SIGTRAN
STP
MGCP/MEGACO
Trunking Gateway
Residential Gateway
Trunking Gateway
Residential Gateway
CO Switch
Trunking Gateway
Residential Gateway
RTP
8
Softswitch

• The switching functions are handled by software
• International Softswitch Consortium (ISC)
• www.softswitch.org
• To promote the softswitch concept and related
  technologies
• Why the softswitch approach is popular?
• A distributed architecture
• For network operators
• It is possible to use different network
  components from different vendors.
• For equipment vendors
• It is possible to focus on one area.

9
Softswitch/PSTN Interworking

• SIP is often used as the signaling protocol
  between the MGCs.

10
Requirements for Media Gateway Control 1/2

• RFC 2895
• Media Gateway Control Protocol Architecture and
  Requirements
• Requirement
• The creation, modification and deletion of media
  streams
• Including the capability to negotiate the media
  formats
• The specification of the transformations applied
  to media streams
• Request the MG to report the occurrence of
  specified events within the media streams, and
  the corresponding actions

11
Requirements for Media Gateway Control 2/2

• Request the MG to apply tones or announcements
• The establishment of media streams according to
  certain QoS requirements
• Reporting QoS and billing/accounting statistics
  from an MG to an MGC
• The management of associations between an MG and
  an MGC
• In the case of failure of a primary MGC
• A flexible and scalable architecture in which an
  MGC can control different MGs
• Facilitate the independent upgrade of MGs and MGCs

12
Protocols for Media Gateway Control

• The first protocol is MGCP
• RFC 2705, informational
• To be succeeded by MEGACO/H.248
• Has be included in several product developments
• MEGACO/H.248
• A standards-track protocol
• RFC 3015 is now the official version.

13
Relation with H.323 Standards
14
H323, SIP MGCP, MEGACO
CA
SS7
PSTN
SG
MGCP
TGW
RGW
GW
GK
TN
CO
GW
GK
TN
PSTN
H.323
RTP
MCU
TN
TN
MCU
TN
TN
CA Call Agent TGW Trunking Gateway RGW
Residential Gateway SG Singling Gateway
GW Gateway GK Gatekeeper TN
Terminal MCU Multipoint Control Unit
15
H323, SIP MGCP/MEGACO

• H.323 , SIP
• peer-to-peer
• internet oriented
• intelligent endpoint
• optional GK
• decentralized
• Problems
• maintenance
• cost scalability of large systems
• signaling media control are coupled
• interoperability with SS7

• MGCP/MEGACO
• client-server
• traditional telephony
• intelligent server
• dumb terminal
• centralized
• Concept
• gateway decomposed
• separate call control from media ports
• CA, MG, SG
• interoperability with PSTN

16
MGCP Connection Establishment
17
MGCP

• A master-slave protocol
• Call agents (MGCs) control the operation of MGs
• Call-control intelligence
• Related call signaling
• MGs
• Do what the CA instructs
• A line or trunk on circuit-switched side to an
  RTP port on the IP side
• Types of Media Gateway
• Trunking Gateway to CO/Switches
• Residential Gateway to PSTN Phones
• Access Gateway
• Communication between call agents
• Likely to be the SIP

18
The MGCP Model

• Endpoints
• Sources or sinks of media
• Trunk interfaces
• POTS line interfaces
• Announcement endpoint
• Connections
• Allocation of IP resources to an endpoint
• An ad hoc relationship is established from a
  circuited-switched line and an RTP port on the IP
  side.
• A single endpoint can have several connections

19
MGCP Endpoints 1/3

• DS0 channel
• A digital channel operates at 64kbps.
• Multiplexed within a larger transmission facility
  such as DS1 (1.544 Mbps) or E1 (2.048 Mbps)
• G.711 (u-law or A-law)
• Analog line
• To a standard telephone line
• An analog voice stream
• Could also be audio-encoded data from a modem
• The gateway shall be required to extract the data
  and forward it as IP packets.

20
MGCP Endpoints 2/3

• Announcement server access point
• Provide access to a single announcement
• One-way
• No external circuit-switched channels
• Interactive voice response (IVR) access point
• Provide access to an IVR system
• Conference bridge access point
• Media streams from multiple callers can be mixed
• Packet relay
• A firewall between an open and a protected
  networks

21
MGCP Endpoints 3/3

• Wiretap access point
• For listening to the media transmitted
• One way
• ATM trunk-side interface
• The termination of an ATM trunk
• May be an ATM virtual circuit

22
Endpoint Identifier

• GWs Domain Name Local Name
• Local Name
• A hierarchical form X/Y/Z
• trunk4/12/7_at_gateway.somenetwork.net
• To identify DS0 number 7 within DS1 number 12 on
  DS3 number 4 at gateway.somenetwork.net
• Wild-cards
• , any , all
• e.g., trunk1/5/_at_gateway.somenetwork.net
• CA wants to create a connection on an endpoint in
  a gateway and does not really care which endpoint
  is used.
• e.g., trunk1/5/_at_gateway.somenetwork.net
• CA requests statistical information related to
  all endpoints on a gateway.

23
MGCP Calls and Connections

• A connection
• Relationship established between a given endpoint
  and an RTP/IP session
• A call
• A group of connections
• The primary function of MGCP is to enable
• The connections to be created
• The session descriptions to be exchanged between
  the connections

24
MGCP Commands

• 9 commands to handle Connection/Endpoints
• EndpointConfiguration (coding characteristics)
• NotificationRequest (requested events)
• Notify (GW detected events)
• CreateConnection
• ModifyConnection
• DeleteConnection
• AuditEndpoint
• AuditConnection
• RestartInProgress (GW taken in/out of service)
• All commands are acknowledged.

EPCF RQNT NTFY CRCX MDCX DLCX AUEP AUCX RSIP
25
MGCP Command Format

• A command line
• Request verb (the name of the command)
• Transaction id
• Endpoint id (for which the command applies)
• Protocol version
• A number of parameter lines
• An optional session description (SDP)
• Separated by a single empty line
• Command Encapsulation
• One command can be included within another
• Only one level of encapsulation
• E.g., when instructing a gateway to create a
  connection, CA can simultaneously instruct the
  gateway to notify the CA of certain events.

26
MGCP Parameters 1/6

• BearInformation (B)
• The line-side encoding
• Bemu
• CallId (C)
• Comprised of hexadecimal digits
• Capabilities (A)
• In response to an audit
• ConnectionId (I)
• Comprised of hexadecimal digits
• ConnectionMode (M)
• Send only, receive only and send-receive

27
MGCP Parameters 2/6

• ConnectionParameters (P)
• Connection-related statistical information
• Average latency, jitter, packets
  sent/received/lost
• GW -gt CA
• DetectEvents (T)
• That an endpoint should detect during quarantine
  period
• E.g., off-hook, on-hook, hook-flash, DTMF digits
• LocalConnectionDescripter (LC)
• An SDP session description
• LocalConnectionOptions (L)
• Bandwidth, packetization period, silence
  suppression, gain control, echo cancellation
• L eoff, son
• To turn echo cancellation off and to turn silence
  suppression on

28
MGCP Parameters 3/6

• EventStates (ES)
• In response to an audit command
• A list of events associated with the current
  state
• MaxMGCPDatagram (MD)
• To indicate the maximum size MGCP packet
  supported by an MG
• Included in the response to an AUEP command
• NotifiedEntity (N)
• An address for the CA
• ObservedEvents (O)
• Detected by an endpoint
• PackageList (PL)
• Supported by an endpoint
• Events and signals are grouped into packages
• Analog line endpoint

29
MGCP Packages

• Group events and signals into packages
• Generic Media (G)
• DTMF (D)
• MF (M)
• Trunk (T)
• Line (L)
• Handset (H)
• RTP (R)
• Network Access Server (N)
• Announcement Server (A)
• The experimental packages have names beginning
  with the two character x-.

Gateway
Supported packages
Trunk GW (ISUP) Trunk GW (MF) Network Access
Server Combined NAS/VOIP GW Access GW
(VOIP) Access GW (VOIP NAS) Residential
GW Announcement GW
G, D, T, R G, M, D, T, R G, M, T, N G, M, D, T,
N, R G, M, D, R G, M, D, N, R G, D, L, R A, R
30
MGCP Parameters 4/6

• QuarantineHandling (Q)
• Events that occur during the period in which the
  GW is waiting for a response to a Notify command
• Process the events or discard them
• ReasonCode (E)
• When a GW deletes/restarts a connection
• RemoteConnectionDescripter (RC)
• An SDP session description
• RequestEvents (R)
• A list of events that an endpoint is to watch for
• Associated with each event, the endpoint can be
  instructed to perform actions
• E.g., collect digits, or apply a signal

31
MGCP Parameters 5/6

• RequestInfo (F)
• In response to audit requests
• The current values of RequestEvents, DigitMap,
  NotifiedEntity
• RequestIdentifier (X)
• To correlate a given notification from a GW
• RestartDelay (RD)
• A number of seconds indicating when an endpoint
  will be brought back into service
• RestartMethod (RM)
• Graceful or Forced

32
MGCP Parameters 6/6

• SecondConnectionId (I2)
• The connection on a second endpoint
• SpecificEndpointID (Z)
• A response to a command using a wild card
• SecondEndpointID (Z2)
• A connection between two endpoints on the same GW
• SignalRequests (S)
• Signals to be applied by an endpoint
• SpecificEndpointID (Z)
• Used to indicate a single endpoint

33
Digit Map

• CA ask GW to collect user dialed digits
• Created by CA
• Usage
• Gateways detect a set of digits.
• e.g., (11x080xxxxxx03xxxxxxx002x.T)
• Match accumulated digits
• under-qualified, do nothing further
• matched, send the collected digits to CA
• over-qualified, send the digits to CA

Inter-digit Timer
34
MGCP Response

• Header
• A response line
• Return code TransID Commentary
• A set of parameter lines (optional)
• E.g., I A3C47F21456789F0 (ConnectionId)
• Session Description
• Session Description Protocol
• separated from header by an empty line

35
Call Setup Using MGCP
36
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37
Call Flow for RGW to TGW
38
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39
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40
Call Flow for RGW to TGW

• RQNT(1) NotificationRequest
• RQNT 1201 hrd3/15_at_rgw.whatever.net MGCP 1.0
• N CA_at_ca.whatever.net5678
• X 0123456789AC
• R hd(E(R(hu(N)),S(dl),D/(D)))
• D (11x080xxxxxx57xxxxx002x.T)
• ACK to RQNT(1)
• 200 1201 OK

N NotifyEntity X RequestIdentifier R
RequestEvents D DigitMap
E Embedded Request R Notification Request N
Notify immediately S Signal Request D Digit Map
41
Call Flow for RGW to TGW

• NTFY(2) Notify from RGW
• NTFY 2002 hrd3/15_at_rgw.whatever.net MGCP 1.0
• N CA_at_ca.whatever.net5678
• X 0123456789AC
• O 5721043
• ACK to NTFY(2)
• 200 2002 OK

N NotifyEntity X RequestIdentifier O
ObservedEvent
42
Call Flow for RGW to TGW

• CRCX(3) CreateConnection
• CRCX 1204 hrd3/15_at_rgw.whatever.net MGCP 1.0
• C A3C47F21456789F0
• L p10, a G.711 G.726-32
• M recvonly
• X 0123456789AD
• R hu
• ACK to CRCX(3)
• 200 1204 OK
• I FDE234C8
• Session Description

C CallId L LocalCXOptions p packetize
period(ms) a Compression Algo. M Mode X
RequestIdentifier R RequestEvents I ConnectionId
43
Call Flow for RGW to TGW

• ACK to CRCX(3) Session Description
• v0
• cIN IP4 140.96.102.166
• maudio 3456 RTP/AVP 0 96
• artpmap96 G726-32/8000

G726G732 encoded audio sample at 8 kHZ
v protocol version c connection information m
media name and transport address a more media
attribute line
44
Call Flow for RGW to TGW

• CRCX(4) CreateConnection
• CRCX 1205 card6/5_at_tgw.whatever.net MGCP 1.0
• C A3C47F21456789F0
• L p10, a G.711 G.726-32
• M sendrecv
• Session Description from ACK(3)
• ACK to CRCX(4)
• 200 1205 OK
• I 32F345E2
• Session Description

C CallId M Mode I ConnectionId
45
Call Flow for RGW to TGW

• MDCX(5) ModifyConnection
• MDCX 1206 hrd3/15_at_rgw.whatever.net MGCP 1.0
• C A3C47F21456789F0
• I FDE234C8
• M recvonly
• Session Description from ACK(4)
• ACK to MDCX(5)
• 200 1206 OK

C CallId I ConnectionId M Mode
46
Call Flow for RGW to TGW

• RQNT(6) NotificationRequest
• RQNT 1207 hrd3/15_at_rgw.whatever.net MGCP 1.0
• N CA_at_ca.whatever.net5678
• X 012345789AE
• R hu
• S v (alerting)
• ACK to RQNT(6)
• 200 1207 OK

N NotifyEntity X RequestIdentifier R
RequestEvents S SignalRequests
47
Call Flow for RGW to TGW

• MDCX(7) ModifyConnection
• MDCX 1209 hrd3/15_at_rgw.whatever.net MGCP 1.0
• C A3C47F21456789F0
• I FDE234C8
• M sendrecv
• X 012345789AF
• R hu
• ACK to MDCX(7)
• 200 1209 OK

C CallId I ConnectionId M Mode
48
Call Flow for RGW to TGW

• DLCX(8) DeleteConnection
• DLCX 1210 hrd3/15_at_rgw.whatever.net MGCP 1.0
• C A3C47F21456789F0
• I FDE234C8
• ACK to DLCX(8)
• 200 1210 OK
• P PS1245, OS62345, PR780, OR45123, PL10,
  JI27, LA48

C CallId I ConnectionId
PS Packets sent OS Octets sent PR Packets
received OR Octets received PL Packets lost JI
Average Jitter (ms) LA Average Latency (ms)
49
Usage of Commands

• NotificationRequest()
• EndpointId,
• NotifiedEntity,
• RequestedEvents,
• RequestIdentifier,
• DigitMap,
• SignalRequests,
• QuarantineHandling,
• DetectEvents,
• encapsulated EndpointConfiguration

Request
Request
Notify
Time
Quarantine Period
Q
T
Q process/discard step/loop (notify) T
events to detect during quarantine
50
Usage of Commands

• CreateConnection()
• CallId,
• EndpointId,
• NotifiedEntity,
• LocalConnectionOption,
• Mode,
• RemoteConnectDescriptorSecondEndpointId
• Encapsulated NotificationRequest,
• Encapsulated EndpointConfiguration

Hairpin Connection
51
Usage of Commands

• DeleteConnection()
• Delete one connection from Call Agent
• ConnectionId in the parameter line
• GW responds with Connection parameters
• Multiple connections from Call agent
• Only EndpontId (and CallId) in the parameter line
• GW does not respond with Connection parameters
• From gateway
• Reason-Code Connection-Parameters in Command

52
Usage of Commands

• RestartInProgress()
• EndPointId
• RestartMethod
• RestartDelay
• Reason-code
• ReturnCode
• NotifiedEntity

• RestartMethod
• Forced
• Restart
• Disconnected
• Graceful

53
Usage of Commands

• AuditEndPoint()
• EndpointId
• RequestedInfo
• ReturnCode
• EndPointIdList
• RequestedEvents
• DigitMap
• SignalRequests
• RequestIdentifier,

If EndpointId is wildcard, EndPointIdList is
returned
54
Usage of Commands

• AuditConnection()
• ConnectionId
• RequestedInfo
• ReturnCode
• CallId
• NotifiedEntity
• LocalConnectionOptions

Wildcard shall not be used
55
Interworking between MGCP and SIP
56
Interworking between MGCP and SIP